How do I reset UI panel zoom level?
Panels and other user interface dialogs can be customized to your personal preference. One example for customization is the horizontal or vertical alignment. Another is zoom factor.

https://wiki.blender.org/index.php/Doc:2.6/Manual/Interface/Panels

The panel focused by the mouse pointer can be zoom in and out by holding Ctrl MMB  and moving the mouse right and left. 

To reset the zoom factor to default, focus the panel with the mouse pointer and press Home.
On a MacBook (Pro), use fn+←

This will reset zoom to the default which is dependent on the DPI settings configured in system preferences:

https://www.blender.org/manual/preferences/system.html